Capacitors are devices for storing electrostatic energy through the separation of electric charges of opposite signs. All capacitors share a common structure of a pair of parallel metallic electrodes or "plates" separated by a layer of dielectric material. The capacitor is "charged" by transferring electric charge from one electrode to the other under the action of an applied potential difference, thus establishing an electric field within the dielectric material. Capacitors used for low voltage operations are typically constructed in the form of stacked flat multilayers of alternating dielectric and electrode sheets. Large capacitors used for high voltage operations are typically constructed by winding together in a cylinder dielectric film interleaved with metal electrode foil.
The dielectric material is a nonconducting medium which can serve to hold an electric field. The medium may be a solid, liquid, gas, or combination of these. A number of materials have been used as dielectrics including paper, mica, high-polymer plastic films (e.g., polypropylenes, polyethylenes, polystyrenes, polyesters, polyimides) and ceramics such as glass and metal-oxide ceramics. Each type of dielectric material has a characteristic dielectric permittivity. This is a measure of the rate of increase of the charge that can be stored in a capacitor with increase in the potential difference (voltage) applied across the electrodes, and is directly proportional to the capacitance.
Capacitors are used for a broad range of electrical applications, such as DC filters in power supplies, timing circuits, coupling or decoupling components, frequency tuning, and energy storage. Energy storage capacitors are designed primarily for use in high-power electrical applications that require delivery of high-power pulses over specific times. The amount of energy that is stored in these capacitors ranges from a few joules to greater than 50 kilojoules. A number of existing industrial and military applications require electrical capacitors having high-energy-density storage and high-power- delivery capabilities. Applications such as pulsed magnetic forming and welding of metals, explosive shearing, or highly penetrating X-ray imaging, for example, require the delivery of repetitive bursts of power in short pulses. Examples of potential future applications include the electrical launch of military projectiles or missiles from mobile vehicles, and auxiliary power for rapid acceleration of electrically-driven automobiles.
In order for high energy and high power applications to be viable in mobile systems, a small sized capacitor is required rather than the large volume capacitor banks currently employed to generate high bursts of power. There is thus a need in the art for a relatively small-sized capacitor that can store and discharge large bursts of electrical energy at high density.
Many of the high energy applications currently employ chemical storage batteries because chemical storage can provide a higher energy density than can be achieved using the electrical energy storage of capacitors. However, electrostatic energy storage is preferable in many cases because an electrical impulse from a charged capacitor can be delivered in a much shorter time and thus at much higher power than can an equivalent amount of energy developed from a chemical storage battery.
In order to increase the storage and delivery capabilities of electrical capacitors, while retaining the small size desirable for new applications, it is necessary to increase the energy density, or the amount of energy stored per unit volume, of the capacitor. This can be accompanied by increasing either (or both) the dielectric permittivity or the maximum tolerable electric field--i.e., the dielectric breakdown strength. Increasing the dielectric permittivity may be achieved in a number of ways, such as increasing the number density and magnitude of dipolar molecules or groups in a dielectric material. The ability of the polar groups or dipoles to become oriented in an electric field contributes to the dielectric properties of a material. U.S. Pat. No. 4,586,111 to Cichanowski, for example, teaches the use of polymers having flexible backbones which allow rotational freedom of the substituents, as a method of increasing the dielectric permittivity of a dielectric material.
It is therefore an object of the present invention to provide dielectric materials suitable for the construction of small, compact energy-dense capacitors capable of delivering large amounts of power.